The present invention relates to apparatus for pilferage control. More particularly, it is directed to apparatus for detecting the presence of a telltale element in an unauthorized zone.
For the purpose of controlling pilferage, it has been proposed heretofore to secure specially constructed tags to the articles to be protected which tags must be deactivated or removed for authorized removal of the articles from the controlled area. In U.S. Pat. No. 3,895,368 issued to Lloyd L. Gordon and Robert D. Williamson for "Surveillance System and Method Utilizing Both Electrostatic and Electromagnetic Fields", and assigned to the same assignee as the present application, there is described apparatus in which a microwave signal generator projects an electromagnetic wave into a space under surveillance to establish a first field. A pulse or frequency modulated low frequency generator is used to apply a voltage to a discontinuous conductor for establishing a second field, electrostatic in nature, throughout the space. Presence in the space of a miniature, passive, electromagnetic wave receptor-reradiator in the form of a semi-conductive diode connected to a dipole antenna causes the reradiation of the low frequency component modulated on the microwave component as a carrier. The front end of a receiver system is tuned to the microwave frequency signal. A coincidence circuit energizes an alarm circuit whenever the detected signal coincides with the original modulation envelope being applied to the low frequency generator. The patent contains a general statement that the discontinuous conductor may be extended across the areaway being protected and that a grounded conductor may be located in the floor in order to provide a return path for the electrostatic signals, if necessary. In a preferred embodiment, described in said patent, pedestals are located on opposite sides of the areaway to be protected which pedestals contain foil elements for establishing the electrostatic field. In a specific example, the foil elements are mentioned as being 4".times.4" in size, and energized by a 245 V RMS signal.
It has been found, however, that when the electrostatic field radiators are located in the side pedestals above the floor level, it is difficult to confine the electrostatic field to the precise area desired to be controlled between the pedestals. When the radiated energy extends beyond the desired boundaries, it is referred to as over-ranging. Over-ranging is undesirable since it cuts down on the floor space adjacent the controlled areaway where tagged articles can be located legitimately or may be transported by someone without tripping an alarm.